1. Field of the Invention
The invention relates to an overvoltage protection element with a housing, with at least one overvoltage limiting component which is located in the housing, especially a varistor or a gas filled surge arrester, and with at least two connection elements for electrical connection of the overvoltage protection element to the current path or signal path which is to be protected, in the normal state of the overvoltage protection element the connection elements each being in electrical contact with one pole of the overvoltage limiting component at a time.
2. Description of Related Art
German Patent Application DE 42 41 311 A1 discloses an overvoltage protection element which has a thermal disconnector for monitoring of the state of a varistor. In this overvoltage protection element, the first connection element is connected via a flexible conductor to a rigid isolating element whose end facing away from the flexible conductor is connected via a solder site to a terminal lug which is provided on the varistor. The other connection element is tightly connected to the varistor or a terminal lug on the varistor via a flexible conductor. The isolating element is exposed to a force from a spring system which leads to the isolating element being moved linearly away from the terminal lug when the solder connection is broken so that the varistor is electrically disconnected when thermally overloaded. When the solder connection is broken a telecommunications contact is actuated at the same time via the spring system, as a result of which remote monitoring of the state of the overvoltage protection element is possible.
German Utility Model DE 20 2004 006 227 U1 and corresponding U.S. Pat. No. 7,411,769 B2 disclose an overvoltage protection element in which the state of a varistor is monitored according to the principle of a temperature switch so that when the varistor overheats a solder connection provided between the varistor and the interrupting element is broken; this leads to electrical isolation of the varistor. Moreover, when the solder connection is broken a plastic element is pushed by the reset force of a spring out of a first position into a second position in which the isolating element which is made as an elastic metal tongue is separated thermally and electrically from the varistor by the plastic element so that an arc which may arise between the metal tongue and the contact site of the varistor is extinguished. Since the plastic element has two colored markings located next to one another, it acts additionally as an optical state display, as a result of which the state of the overvoltage protection element can be read directly on site.
German Patent DE 699 04 274 T2 likewise discloses an overvoltage protection element with a thermal disconnection mechanism. In this overvoltage protection element one end of a rigid, spring-loaded slide in the normal state of the overvoltage protection element is soldered to the first connection element and also to the terminal lug which is connected to the varistor. Unacceptable heating of the varistor here also leads to heating of the solder site so that the slide is pulled out of the connection site between the first connection element and the terminal lug as a result of the force of a spring acting on it; this leads to electrical disconnection of the varistor.
German Patent DE 695 03 743 T2 discloses an overvoltage protection element with two varistors, which has two isolating means which can disconnect the varistors each individually on their live end. The isolating means each have an elastic isolating tongue, the first end of the isolating tongue being tightly connected to the first terminal and the second end of the isolating tongue being attached to a connecting tongue on the varistor in the normal state of the overvoltage protection element via a solder site. If unacceptable heating of the varistor occurs, this leads to melting of the solder connection. Since the isolating tongue in the soldered-on state (normal state of the overvoltage protection element) is deflected out of its rest position and is thus pretensioned, the free end of the isolating tongue springs away from the connecting tongue of the varistor when the solder connection softens, as a result of which the varistor is electrically disconnected. In order to ensure the required insulation resistance and resistance to creepage and to extinguish an arc which forms when the isolation site opens, it is necessary that when the isolating tongue is pivoted a distance as great as possible between the second end of the isolating tongue and the connecting tongue of the overvoltage limiting component is achieved.
The known overvoltage protection elements are generally made as “protective plugs” which together with a lower part of the device form an overvoltage protection device. For installation of such an overvoltage protection device which, for example, is designed to protect the phase-carrying conductors L1, L2, L3 and the neutral conductor N and optionally also the ground conductor PE, in the known overvoltage protection devices there are the corresponding terminals for the individual conductors on the lower part of the device. For simple mechanical and electrical contact-making of the lower part of the device with the respective overvoltage protection element, in the overvoltage protection element the connection elements are made as plug pins for which there are corresponding receptacles which are connected to the terminals in the lower part of the device so that the overvoltage protection element can be slipped onto the lower part of the device.
In these overvoltage protection devices, the installation and mounting can be done very easily in a time-saving manner by the intermateability of the overvoltage protection elements. In addition these overvoltage protection devices in part have a changeover contact as a primary detector for remote reporting of the state of at least one overvoltage protection element and an optical state display in the individual overvoltage protection elements. It is displayed via the state display whether the overvoltage limiting component located in the overvoltage protection element is still serviceable or not. Here, especially varistors are used as the overvoltage limiting component, but depending on the purpose of the overvoltage protection element also gas-filled surge arresters, spark gaps or diodes can be used.
The above described thermal disconnection devices which are used in the known overvoltage protection elements and which are based on the melting of a solder connection must perform several tasks. In the normal state of the overvoltage protection element, i.e., in the unisolated state, a reliable and good electrical connection must be ensured between the first connection element and the overvoltage limiting component. When a certain boundary temperature is exceeded the isolating point must ensure reliable disconnection of the overvoltage limiting component and continuous insulation resistance and resistance to creepage. But, the problem is that the solder connection is continuously loaded with a shear stress in the normal state of the overvoltage protection element as a result of the spring force of the spring element or the isolating tongue which has been deflected out of its rest position.